Novel analogs of the endogenous morphinoid pentapeptides, the enkephalins, are being synthesized and will be evaluated pharmacologically for narcotic agonist and antagonist activities in vitro by the guinea pig ileum assay and in vivo by the mouse tail-flick test. The research is designed to probe: a) requirements as to intramolecular hydrogen bonding and conformation for the enkephalins to bind to opioid receptors and to display opiod properties; and b) possibilities for developing metabolically stable agents showing useful durations of pharmacologic action.